1. Field of the Invention
The present invention relates to an antenna device, an RFID tag, and a communication terminal apparatus which are preferably used for near field communication.
2. Description of the Related Art
An RFID system in which a reader/writer and an RFID (Radio Frequency Identification) tag communicate with each other by a contactless scheme to transmit information between the reader/writer and the RFID tag is proliferating. For RFID systems, an HF-band RFID system using the 13 MHz band and a UHF-band RFID system using the 900 MHz band are commonly used. In particular, for an RFID system for item management, a UHF-band RFID system is a promising system in terms of its long communication distance and capability to perform reading and writing multiple tags at a time.
An RFID tag used in an RFID system includes an RFIC chip for processing radio signals and an antenna element for transmitting and receiving radio signals. An RFID tag for the UHF band uses a dipole antenna or a loop antenna as an antenna element. In particular, the loop antenna is useful as an antenna for a compact RFID tag because, although it is a magnetic field radiation type antenna and thus has a slightly short communication distance, it can achieve miniaturization.
In the case of using a loop antenna, in a situation in which an open surface of the loop antenna faces a metal body, if an affixation target item is a metal body, for example, eddy current that cancels out magnetic field changes in the loop antenna is generated in the metal body, which makes it difficult to secure a sufficient communication distance.
To solve the problem of eddy current, as described in, for example, Japanese Patent Unexamined Publication No. 2004-166175 bulletin or the like, there is known a technique for interposing a magnetic material such as ferrite between a loop antenna and an affixation target.
FIG. 1A is a perspective view of an antenna for a reader/writer shown in Japanese Patent Unexamined Publication No. 2004-166175 bulletin, and FIG. 1B is a cross-sectional view of an A-A′ portion of FIG. 1A. In the antenna for a reader/writer, a soft magnetic material 5 is disposed between an antenna coil 4 whose both ends are connected to an electronic circuit 8 and a metal surface 6. As shown in FIG. 1B, magnetic fluxes 9 generated in the antenna coil 4 pass through the soft magnetic material 5 and do not reach the inside of the metal surface 6 almost at all. Thus, a change in resonance frequency and an increase in loss caused by eddy current generated in the metal surface 6 are suppressed.
However, in the antenna having the structure shown in FIGS. 1A and 1B, it is difficult to sufficiently block entry of magnetic fluxes into a metal body, depending on the used frequency band, the thickness of the magnetic material, or the like. That is, to suppress the influence of the metal body, the magnetic material needs to be thickened, which in turn increases the sizes of an antenna device and an apparatus including the antenna device.